this is the difference0dB for headunit/least distortion
-5dB for amp gains/sq setting << what i use
-10dB for amp gain/compromise << what most use
-15dB for amp gain/spl setting << too quiet for rap music
max would be volume 34 in this case
"-15 too quiet for rap music"? i find a break in your logic here. tuning with -15dB (track 7) at the head unit and using a dd1, would mean that your turning your amp up higher before getting distortion, so obviously when you play a regular cd it will be louder than if it was tuned with -5 (track 3) or -10 (track 5). what would be too quiet would be tuning with track 3. that is unless your going strictly for quality but i like my shit pounding loud.
You never use -15dB for the headunit, you always use 0dB to get the cleanest highest voltage possible. Rap music bass is louder than -15dB, thus anything louder than the tone used to set the dd-1 on sub amp will push the amp into clipping. Most rap music is between -5dB and -10dB, so using -15dB means most of the time you are +5dB into clipping. There is no break in logic besides yours.
Yes I agree you only use 0dB to find your max volume. when I said "at the head unit" I meant which track I'm playin to set amp gains only. I get what your saying though but in your OP you said -15dB is too quiet for rap - now are we talking about what rap actually plays at or are we talking about setting gains with dd1? If we're talking about setting gains then the -15dB track would cause it to be too loud, as you stated as being +5dB correct?